1. Technical Field
This invention relates to systems for processing of semiconductor wafers. In particular, this invention is directed to a technique for removing excess resist material on a semiconductor wafer surface.
2. Prior Art
In the manufacture of semiconductor wafers, a resist is commonly applied in blanket form and then exposed, developed and selectively removed for purposes of patterning various areas on the wafer. Frequently, the resist itself remains over alignment marks which are provided in peripheral portions of the wafers and within the chips themselves. Additionally, during device processing, residual portions of the resist may exist, that is not fully removed. The presence of resist over alignment marks introduces complications and errors in detection of those points which in turn leads to alignment errors in the fabrication of integrated circuit devices. In the case of excess material existing on the active area of the wafer, such contaminant may lower yield output of the manufacturing process and frequently introduce additional cleaning and inspection steps which are expensive and time-consuming.
Current manufacturing techniques may employ separate discrete processing steps to remove the resist over the alignment marks and then continue processing. Generally, this removal step is chemically oriented. However, depending on the resist which is used, chemical processing may or may not be achievable. For example, use of a negative photo resist precludes chemical processing since such processing would render all of the resist useless for further processing.
Moreover, the thickness of the resist over, alignment marks is significant as a function of a lithographic system which is used. For example, in the case of E-beam lithography using a 25 kv accelerating voltage, resist thickness in the range of 3-4 microns will prevent the E-beam from returning through the resist from the alignment marks. In the case of optical alignment systems, the resist presents problems in viewing for purposes of achieving accurate alignment. Thus, in any given processing scheme the problem of having usable alignment marks exists. However, no system approach is known to expose such marks by removing overlying resist while not further contaminating the structure.
Within the prior art, there is a recognition that lasers may be used for purposes of performing scribing, cutting, or like task. Typical is U.S. Pat. No. 3,742,183 which illustrates a laser cutter having a scoop connected to a suction pipe for purposes of removing by-products of the laser burning process. Laser machining for purposes of boring holes, scribing or the like, is disclosed in U.S. Pat. Nos. 4,032,743, 4,078,165 and 4,267,427.
Reference is made to U.S. Pat. Nos. 3,991,296 and 4,114,018 which are directed to ablative techniques employing lasers and formation of grooves in a semiconductor wafer by the use of a laser beam. While such technology recognizes a myriad of uses of lasers, none is specifically directed to the removal of a resist over alignment marks and to the removal of excess material existing on a semiconductor substrate.
Within the laser scribing technology, a myriad of systems also exist which incorporate suction techniques to remove entrained gases, vaporized materials or the like which are generated as a by-product of laser processing. U.S. Pat. No. 3,866,398 illustrates the use of a laser directed onto a silicon slice to be scribed. The laser is directed through a mirror which is placed on a scavenging system in alignment with the slice or kerf to be generated in the silicon wafer. The scavenging system includes a hood having a vacuum port. Perforated tubing is used to introduce a reagent gas which combines with the high thermal energy gaseous silicon to form a gaseous silicon component which is drawn out of the chamber through the vacuum system.
U.S. Pat. No. 4,347,785 also employs a vacuum chamber for purposes of removing plasma and debris which are formed as a consequence of laser scribing.
Less relevant is U.S. Pat. No. 3,524,038 which discloses use of an adjacent vacuum to remove debris created during cutting of metal and an analogous system in IBM TDB, Vol. 14, No. 3, page 709, August 1971 vis-a-vis scribing of a silicon wafer.
Thus, while the prior art is replete with a number of schemes employing laser technology to achieve scribing or cutting, none are uniquely suited for the removal of resist material which may exist over an alignment mark or, the removal of discrete areas of resist which constitutes excess material on a semiconductor wafer surface. A key problem is the inability in the prior art to define a system that removes material covering alignment marks without damaging the alignment marks themselves. Another problem is that the laser removal of resist requires ablative photodecomposition.
The use of a excimer laser to remove resist by ablative photodecomposition has been proposed in R. Srinivasan et al, J. Am. Chem. Soc., 104, 6784 (1982). However, the use of such a laser to remove resist introduces another problem, the removal of the resulting particulate contaminants. The mere use of a vacuum to remove particulate contaminants will generally be ineffective unless some technique to prevent spreading at the point of ablative decomposition is employed. None of the prior art identified here are capable of such containment.